U.S. patent number 5,633,494 [Application Number 08/532,327] was granted by the patent office on 1997-05-27 for fiber optic bending and positioning sensor with selected curved light emission surfaces.
Invention is credited to Lee Danisch.
United States Patent |
5,633,494 |
Danisch |
May 27, 1997 |
Fiber optic bending and positioning sensor with selected curved
light emission surfaces
Abstract
A curvature or bending and displacement sensor is composed of a
fiber optic or light wave guide, for attachment to a member or
members being curved or displaced. Light is injected at one end and
detected at the other end. Curvature of the fiber results in light
loss through an emission surface or surfaces, sometimes in
conjunction with a superimposed curvature in a plane other than
that of the curvature to be measured, this loss being detected. The
loss of light detection is used to produce indication of curvature
or displacement. The light emission surfaces extend in various
forms, such as a surface strip or band. Particularly, in an
example, the emission surfaces extend in a substantially peripheral
direction, or in a substantially curved axial direction when in a
curved portion of a curved guide. The placement, shape and
configuration of the emission surfaces allows adjustment of the
linear range of measurement, the overall throughput of light, and
the length over which curvature is measured. Two or more light
guides can be oriented to given indication of direction of
curvature or displacement.
Inventors: |
Danisch; Lee (Fredericton, New
Brunswick, CA) |
Family
ID: |
27398666 |
Appl.
No.: |
08/532,327 |
Filed: |
September 22, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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234953 |
Apr 28, 1994 |
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915283 |
Jul 20, 1992 |
5321257 |
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738560 |
Jul 31, 1991 |
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Current U.S.
Class: |
250/227.16;
250/227.14; 385/13 |
Current CPC
Class: |
G02B
6/02057 (20130101); G01D 5/35377 (20130101); G02B
6/2852 (20130101) |
Current International
Class: |
G01D
5/353 (20060101); G01D 5/26 (20060101); G02B
6/28 (20060101); H01J 005/16 () |
Field of
Search: |
;250/227.16,227.14,227.24,227.31 ;385/32,39,48,13 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Le; Que
Attorney, Agent or Firm: Paul Sharpe, McFadden, Fincham
Parent Case Text
This is a continuation-in-part of application Ser. No. 08/234,953
filed Apr. 28, 1994, abandoned, which in turn is a
continuation-in-part of application Ser. No. 07/915,283 filed Jul.
20, 1992, now issued U.S. Pat. No. 5,321,257, which in turn is a
continuation in part of application Ser. No. 07/738,560, filed Jul.
31, 1991 (now abandoned).
Claims
I claim:
1. A fiber optic bending and position sensor comprising a fiber
optic light guide having a peripheral surface and having at least
one curved light emission surface extending for part of the length
of said light guide, said at least one curved light emission
surface extending at said peripheral surface in a direction
selected from (a) extending around said peripheral surface, (b)
extending in an axial direction along said peripheral surface at a
curved portion of said light guide and including means for
injecting a light beam into an end of said light guide and means
for detecting said light beam after it has passed the light
emission surface zone.
2. A sensor as claimed in claim 1, wherein the light emission
surface or surfaces are positioned to give a maximum change in
light intensity transmitted through the fiber optic light guide
when said sensor is bent in a selected plane.
3. A sensor as claimed in claim 1 including means for measuring the
difference in intensity of said light beam between said one end and
said other end of said light guide.
4. A sensor as claimed in claim 3 including display means for
indicating any said difference in intensity of said light beam as a
bending or displacement of said light guide.
5. A sensor as claimed in claim 1 including a further fiber optic
light guide positioned alongside said fiber optic light guide, said
further light guide having an unbroken cladding layer and forming a
reference light guide.
6. A sensor as claimed in claim 1 having said at least one curved
light emission surface covered by a light absorbent material.
7. A sensor as claimed in claim 1 comprising a fiber optic light
guide in the form of a loop, the loop having said curved light
emission surface therein.
8. A sensor as claimed in claim 7, wherein the loop has a plurality
of curved light emission surface regions therein.
9. A sensor as claimed in claim 8, wherein said curved light
emission surface regions in the curved loop comprise
peripherally-oriented bands grouped on an inside or concave portion
of the curvature of the loop.
10. A sensor as claimed in claim 9 having three to five
peripherally-oriented bands grouped together, with the bands each
extending for about 60.degree. to about 90.degree..
11. A sensor as claimed in claim 8, wherein said curved light
emission surface regions in the curved loop comprise
axially-oriented bands with their axial edges substantially in a
plane parallel to the planes described by the uppermost and
lowermost reaches of said loop and following the curvature of the
loop.
12. A sensor as claimed in claim 11 having from two to six curved
bands in parallel planes.
13. A sensor as claimed in claim 7, wherein said light emission
surface is positioned to gain a maximum change in light intensity
transmitted through the fiber optic light guide when said sensor is
bent in a selected plane.
14. A sensor according to claim 1, wherein the sensor is a
suspension sensor comprising a flexible beam, said fiber optic
bending and position sensor mounted on said flexible beam.
15. The sensor of claim 1 in which at least a portion of said fiber
optic light guide is in the form of a single fiber which serves
both to illuminate said at least one light emission surface and to
collect illumination which has passed said at least one light
emission surface, the illumination and collection end or ends of
the fiber being located in a single region remote from the light
emission surface or surfaces.
16. The sensor of claim 15, wherein the single fiber forms a loop,
at least a portion of the light emission surface is in the loop,
and the apex of the loop is at the distal extremity remote from
said single region.
17. A fiber optic bending and position sensor comprising a fiber
optic light guide having at least one curved light emission surface
extending for part of the length of said light guide, said at least
one curved light emission surface comprising at least one band
extending peripherally of said light guide including means for
injecting a light beam into an end of said light guide and means
for detecting said light beam at the other end of said light
guide.
18. A sensor as claimed in claim 17 having the light emission
surface in the form of a plurality of spaced apart
peripherally-oriented bands covered with a light-absorbing
material.
19. A fiber optic bending and position sensor comprising a fiber
optic light guide having at least one curved light emission surface
extending for part of the length of said light guide, said curved
light emission surface comprising at least one band extending
axially at a curved portion of said light guide including means for
injecting a light beam into an end of said light guide and means
for detecting said light beam at the other end of said light
guide.
20. A sensor as claimed in claim 19 wherein said fiber optic light
guide has a rectangular cross-section, with said at least one
curved light emission surface being on at least one side of said
cross-section at a curved portion of said light guide and each said
emission surface being covered with a light-absorbing material.
21. A sensor as claimed in claim 20, said rectangular cross-section
being oblong and including two spaced parallel wide sides, said at
least one curved light emission surface on one of said wide
sides.
22. A sensor as claimed in claim 21, wherein said light guide is
bent into a U-shape on a plane parallel to planes of said wide
sides, said bend forming said curved portion.
23. A sensor as claimed in claim 21, said light guide being bent
into a U-shape in a plane normal to said planes of said wide sides,
said bend forming said curved portion.
24. A sensor as claimed in claim 23 with a light emission surface
on each of said wide sides, at said bend.
25. A sensor as claimed in claim 19 wherein said fiber optic light
guide has a D-shaped cross-section, having a flat surface on one
side, said light emission surface being formed on said flat
surface.
26. A method of sensing curvature and displacement of an elongate
member, comprising:
attaching a fiber optic light guide to said elongate member, said
light guide having a peripheral surface and having a curved light
emission surface on said peripheral surface and extending for part
of the length of said light guide, said curved light emission
surface extending in a direction selected from (a) extending
peripherally around said peripheral surface, (b) extending axially
along said peripheral surface at a curved portion of said light
guide;
injecting a light beam into one end of said light guide;
detecting said light beam at the other end of said light guide;
measuring the difference in intensity of said light beam between
said one end and said other end; and
indicating bending, or displacement, of said elongate member.
27. The method of claim 26 including attaching a plurality of fiber
optic light guides to said elongate member, each light guide having
a plurality of light emission surfaces, said light emission
surfaces on each individual light guide being positioned to give a
maximum change in light intensity transmitted through said
individual light guide when said elongate member is bent in a
unique selected plane.
28. The method as claimed in claim 27 utilizing two to six fiber
optic light guides, said surfaces oriented in different directions
at predetermined angles relative to each other.
29. The method as claimed in claim 26 including attaching a further
fiber optic light guide alongside said fiber optic light guide,
said further fiber optic light guide acting as a reference light
guide.
30. The method of claim 26 which includes selecting the size, shape
and orientation of each of the emission surfaces of the light guide
so as to optimize sensing of curvatures, including curvature
changes that produce increasing transmission of light with
increasing curvature.
31. The method of claim 26 which includes the use of shear elements
or springs between at least one end portion of said elongate member
and a moveable member, the displacement of which it is desired to
be measured, to communicate displacement to the elongate member,
such that curvature of said elongate member maximally represents
the moveable member displacement to be measured.
32. A fiber optic bending and position sensor comprising, a fiber
optic light guide in the form of a loop having a tight curve
portion, with at least one curved light emission surface
substantially on said tight curve portion, the size and positioning
of the light emission surface being selected to increase
sensitivity to deformation, means for injecting a light beam into
one end of said light guide and means for detecting the light beam
after it has passed the light emission surface zone.
33. The sensor of claim 32 wherein the light emission surface
covers only part of the circumference, is axially-extending and
covered by a light-absorbing material, the tight curve portion
forms a semi-circular shape, and a deformation to be sensed tends
to deform or deflect the semi-circular shaped portion.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
Various methods exist for measuring strain, curvature or
displacement of materials or structural members. One well-known
method is to measure stress on or in the members using resistive
strain gauges arranged on the surface in patterns such that the
bending can be inferred from a knowledge of the modulus of
elasticity of the member. Under some conditions it is advantageous
to measure stress or deformation using optical fibers. Fibers are
ideal for many applications because they can be relatively inert to
environmental degradation, are light in weight, are not affected by
electromagnetic interference, carry no electrical current, and can
be very small and flexible, thus having little or no effect on the
structure in which they are embedded. It is possible to either
cement fibers to surfaces or to embed them inside, such as in
fiber/epoxy composites, concretes, or plastics.
Many types of optical fiber sensors have been developed for the
measurement of stress and position. Most employ interference
techniques to measure changes in length or bend radius of the
fiber. Most of these techniques rely on detecting standing waves
set up in the fiber by reflecting part of the light back from its
distal end. These techniques are very sensitive (comparable to
strain gauges) but require complex and expensive measurement
techniques such as interferometry or optical time domain
reflectometry (OTDR) for their execution. Measurements are very
sensitive to changes in temperature, requiring elaborate
compensation techniques. Another limitation of many of the
interference techniques is insensitivity to direction because the
measurement is made by counting the number of interference peaks
due to distortion of a fiber. Thus, for example, shortening of the
fiber is indistinguishable from elongation, or bending up is the
same as bending down; unless the fibers are arranged in appropriate
curves or other special geometric arrangements.
Equipment for performing interference measurements tends to be
bulky and expensive, requiring frequent adjustment. It must be
capable of distinguishing peaks at spacings of the order of 0.5 to
1 micron or less. This has limited most fiber optic stress
measurements to tests which can be performed under carefully
controlled laboratory conditions.
Non-interference techniques can be used to measure bending in fiber
optics. It is well known that light leaks out of the core of an
optical fiber if it impinges on the cladding at a sufficiently
large angle with respect to the long axis of the fiber. For every
fiber, there is a critical angle dependent on the indices of
refraction of core and cladding, beyond which light will escape. If
the fiber is bent, some of the light in the core will exceed this
angle and escape. This effect has been used to build "microbending"
sensors, which simply measure the percentage of transmission of
light down a fiber. These suffer from relative insensitivity
(little light is lost) at small angles. Usually a microbend sensor
consists of a fiber placed in a corrugated fixture such that a
force applied to the fixture will create many sharp bends in the
fiber. Microbend sensors are used to measure pressures, forces, and
displacement. These sensors also do not measure the direction of
the force unless pre-tension is applied.
Other fiber optic sensors have been constructed in which the
cladding is removed from the core, or the cladding and some of the
core are etched away. These sensors may be more sensitive to
bending than untreated fibers, but, like other bending sensors
mentioned above, give no information about the direction of a bend
unless they are bent at rest. They are thus unsuitable for
incorporating in a simple manner in composite structures containing
many parallel fibers with sensory and structural properties.
Other fiber optic sensors have been constructed which use thin
films in place of the cladding, to give location information based
on the wavelength of the filter produced by the thin film. This
technique shows no improvement in sensitivity over other fiber
optic sensing techniques, so interference techniques must be used
to obtain useful outputs.
Many sensors are based on measurements of strain, which is
basically an elongation of material. Although it is possible to use
multiple strain gauges to infer curvature from strain, it is more
desirable in many circumstances to measure curvature directly.
Often, it is desirable to mount a sensor near the neutral axis of a
beam, where there is no strain associated with curvature of the
beam. Often, curvature is the parameter of direct interest, such as
when measuring deviation from straightness in a pipe or rod. It is
also frequently desirable to measure displacement between two
structures, which can be inferred from the curvature of a flexible
beam or fiber connecting them. Just as strain gauges can be used to
infer curvature in some circumstances, curvature sensors can be
used to infer strain.
Strain gauges have found wide application in a huge variety of
measurement tasks; curvature sensors potentially have just as many
applications. The following examples cover only some of the
potential applications for fiber optic curvature and displacement
sensors: measuring flutter and deflection in aircraft wings and
aerospace truss structures; measuring deflections on cranes and
lifting devices; measuring movement of bridges, dams, and buildings
due to earthquakes, settlement, or other degradation; measuring sag
and deflections of pipes, rods, cables, and beams; measuring the
effects of frost heave on roadways and runways; sensing traffic
movements and soil settlement; measuring wind forces on masts and
towers; sensing parameters of sports equipment including skis,
poles, shoes, fishing equipment, swords, bats, clubs, balls and
clothing; measuring deflections on marine equipment including
masts, spars, cables, hull plates, struts, and booms; measuring
curvatures of vanes, wires, poles and other flexible structures or
probes to infer fluid or slurry flow, speed, and direction of
movement; measuring vibration and sound levels by means of flexing
beams, fibers, or diaphragms; measuring pressures by sensing the
curvature of diaphragms or tanks; measuring acceleration in
general; measuring deceleration and associated forces for the
deployment of airbags; measuring the deflection of support
structures to infer applied weight, forces, torques, and
deflections; forming multi-degree-of-freedom force and torque
sensors; forming input devices for computers including joysticks,
keyboards, and levers; measuring joint angles and deflections on
robots, automatically guided vehicles, automobiles, trucks, tanks,
earth moving equipment, loaders, cranes, ships, airplanes,
helicopters, and spacecraft; measuring the deflections of tire
treads and other rubber or elastomeric moving parts; measuring door
and wheel positions; measuring pedal, vane, rudder, lift surface,
and valve positions; measuring shaft and knob angles, rotations,
and positions; measuring liquid levels by deflection of floats or
bladders; measuring alignment of automotive, marine, or industrial
equipment; measuring positions and motion of reclining seats,
chairs, beds, and medical fixtures; instrumenting medical tools;
instrumenting prosthetic devices; measuring deflections in the
presence of high magnetic fields; measuring magnetic and electric
fields by virtue of forces or motion generated in magnetic or
electric media attached to a curvature sensor; measuring
concentration or presence of liquids, gases, and vapours by virtue
of dimensional changes induced in a substrate to which the
curvature sensor is attached; measuring temperatures by virtue of
dimensional changes induced in a substrate to which the curvature
sensor is attached; measuring positions and angles of parts of
animal, including human, bodies; and many others.
Generally, fiber optic sensors that must be exposed to harsh
environments or that must be embedded, should be intrinsic sensors,
that is, sensors that do not rely on light leaving and then
reentering the fiber. Thus, sensors that involve light exiting a
fiber, reflecting off a surface, and re-entering are not desirable
for many purposes because the surfaces may become contaminated,
thus changing the light intensity.
A wide variety of intrinsic fiber optic sensors has been described,
most of them based on interference techniques. Interference-based
sensors, which rely on mechanical changes to the fiber dimensions
producing changes in light interference patterns within the fiber,
are very sensitive to strain but also to temperature and involve
complex and expensive electronic circuitry. Another drawback is
that usually lasers or laser diodes must be used as light sources
with these sensors, thereby limiting their durability and
longevity, and increasing their cost.
Optical fibers transmit light by virtue of total internal
reflection. The light is contained in a core of transparent
material. Generally, this core is covered with a cladding layer
that has a lower index of refraction than the core. Because the
index of the cladding is lower than that of the core, rays within a
certain range of angles of incidence with the core/cladding
boundary will be refracted back into the core upon striking the
cladding. If the fiber is bent in a curve, small amounts of light
are lost due to changes in the angle of incidence at the curved
core/cladding boundary. If the curvature becomes substantial,
significant amounts of light may be lost. Fibers with a discrete
core/cladding boundary are called step index fibers. Other fibers
called graded index fibers do not have a distinct boundary between
core and cladding, but exhibit a continuous decrease in index of
refraction toward the outer circumference of the fiber. For
simplicity, this description will use terminology consistent with
step index fibers, but graded index fibers may be similarly
treated, as may other light guides including guides of non-circular
cross section including, for example, a D-shaped cross-section,
rectangular and other polygonal cross-sections, and of guides with
gas or liquid surrounds instead of conventional solid cladding. It
is also possible to use metal-covered fibers.
"Microbending" sensors are designed to take advantage of this loss
mechanism. They generally involve a mechanical structure such as a
serrated plate that presses on the fiber, producing a series of
substantial local curvatures (bends). The loss of light is used as
a signal to indicate displacement of the mechanical structure.
Microbending sensors generally do not have a linear loss of light
energy in response to changes in curvature, and are otherwise
undesirable because of the necessity for a mechanical structure,
and the strain which it imposes on the cladding and core of the
fiber during deflection. If fibers are used without a mechanical
structure to translate displacement into large local curvature,
then the light loss due to bending is sufficiently large to be of
practical use only when the bending is large. For small bends, such
unenhanced microbending sensors are not useful because inadvertent
bending of the fiber optic leads carrying light to and from the
sensor portion of the fiber will produce changes in light loss that
are indistinguishable from those produced by bends of the sensor
portion. For these reasons, microbending sensors are generally not
used in embedded applications, and rarely are used for measurements
of curvature.
It is possible to treat optical fibers so that the amount of light
travelling through the core changes more than usual with changes in
curvature. Methods generally involve modification of the cladding
so that it loses more light than usual over a short length. When
straight, more light than usual is lost over the treated zone. When
bent, additional amounts of light are lost due to the greater
interaction of the treated sides with the light travelling through
the fiber. Methods of treatment include abrasion, etching, heat
treatment, embossing, and scraping of the cladding. Such treatment
can produce a loss of light that is linear with curvature over a
wide range, and which is much greater, by orders of magnitude, than
the loss produced by microbending or by inadvertent bending of the
leads carrying light to and from the sensitized zone.
A drawback of the above method of treatment is that loss is
introduced even for a straight fiber, and the modification of the
cladding can weaken glass fibers, especially if it involves removal
of cladding around the entire circumference of the fiber.
It is undesirable to produce excessive light losses. If loss
through the fiber is minimized, it is possible to use an
inexpensive light emitting diode as a source of light, and to use
inexpensive photodetectors and amplifiers to detect the amount of
light being transmitted through the sensor. For this reason, it is
desirable to make sensors with as little loss as possible when at
the maximally transmissive end of their range (low residual light
loss), but with as large a loss as possible due to a change in
curvature (high sensitivity). Preferably, the loss should be a
linear function of curvature, with the centre of the linear range
being at the centre of range of the mechanical quantity (such as
curvature or displacement) being measured. These requirements often
cannot be met with known sensors, because parameters such as
residual light loss and sensitivity cannot be varied independently.
For instance, sensitivity to curvature increases as the length of
the treated zone is increased, but so does residual light loss.
Harvill et al. (U.S. Pat. No. 5,097,252) have described intrinsic
fiber optic sensors with the upper surface of the fiber treated to
sense bending of fingers and other body parts. Although a monotonic
output is claimed, the range of which includes a straight (zero
curvature) sensor, the output is not linear and the range is not
centred about zero curvature. Danisch (U.S. patent applications
Ser. No. 07/738,560 filed on Jul. 31, 1991, and entitled Fiber
Optic Bending and Positioning Sensor and Ser. No. 07/915,283 filed
on Jul. 20, 1992, and having the same title, each naming Lee
Danisch as the inventor, and further described in "Bend-enhanced
Fiber Optic Sensors," SPIE: The International Society of Optical
Engineering, L. A. Danisch, Volume 1795, 204-214, September, 1992,
Boston, Mass., U.S.A.; "Smart Bone," Final Report for Canadian
Space Agency Contract 9F006-1-0006/01-OSC, L. A. Danisch, 24 pp.,
June, 1992; and "Smart Wrist," Final Report for Canadian Space
Agency Contract 9F006-2-0010/01-OSC, L. A. Danisch, March, 1993 has
described fiber optic sensors with a surface of the fiber treated
to emit light at a side with a minimal loss of throughput by means
first described in U.S. Pat. No. 4,880,971, also in the name of Lee
A. Danisch. The Danisch prior art includes descriptions of linear
responses for a wide range of curvatures, a response that drops off
as a cosine function for bends in planes not in the plane of
maximum sensitivity, and a range centred about zero curvature.
Another feature is a light absorbing coating which reduces or
eliminates extraneous responses, including non-linearity. Control
over the positioning of the centre of the range would open the
possibility of mainly using the portion of the range with the
highest light throughput (lowest loss), rather than that with the
lowest throughput (highest loss) as taught in Harvill. This would
be especially useful if the centre could be adjusted without
affecting the residual light loss or sensitivity, or adversely
affecting the strength of the fiber. The prior art does not teach
how this can be done. The prior art describes sensors for which it
is possible to vary the length, width and shape of a single treated
strip, or the depth of multiple notches. Danisch, (U.S. Patent
filings above) describes long sensors ". . . formed by alternating
lengths of fibers with an emission strip with lengths of fully clad
fibers." However, it is not shown how this technique can be used to
gain control over the placement of the centre of the linear
range.
A complicating factor in the manufacture of treated fiber optic
sensors is that if their response to curvature is maximum in a
given plane due to treatment not including the entire circumference
of the fiber, then it can be difficult to maintain a proper
orientation of the plane of maximum sensitivity after treatment but
before embedment of the fiber. The main problem is the ease with
which the fiber can twist about its long axis due to torques
applied at any point along the length of the fiber. This is a
problem for any fiber whose complete circumference is not treated,
including fibers that are treated at both the top and bottom, thus
having a response characteristic that does not distinguish upward
from downward bends, but that distinguishes (through a cosine law)
between up/down and left/right bends.
Another complicating factor in the design of many intensity-based
fiber optic sensor system is the need for a return path and a means
of reflecting or turning the light at the end of the fiber run.
To eliminate the need for a turnaround and return path, a coupler
is often used at the measurement end, such that light can be
injected into a single fiber with a reflector at the end. Injected
light travels through the treated portion of the fiber, is
reflected at the end, and returns to the coupler in the measurement
system in the same fiber. The coupler is designed to extract the
return light only, passing it on to a photodetector and amplifier.
Unfortunately, the coupler introduces large losses and can be
expensive to manufacture. Also, the reflective structure at the end
can be lossy and difficult to manufacture.
In other cases, it is acceptable to use a return fiber with a
reflective structure placed near the ends of the sensor and return
fibers, whose distal ends face or are inserted into the reflective
structure. Such a solution generally involves light leaving and
re-entering the fibers, so that the sensor is no longer an
intrinsic one, or it involves losses that may be unacceptable. It
also invariably involves a reflective structure that is larger than
the diameter of a single fiber or even two fibers, and is thus
unacceptable for embedment.
A disadvantage of a turnaround loop at the distal end of a fiber
optic sensing system is that even if sufficient width is available
for the turnaround, it requires adding extra length to the system
beyond the location of the sensor. This increases the size of the
system and prevents sensing at the distal end of the system. For
instance, it may be desirable to measure changes in curvature at
the top end of a non-hinged but flexible lever which is being used
as a "joystick" form of input device for a computer. If a
turnaround loop is used for a fiber that enters the lever at the
bottom, it would normally be at the top of the lever, thus not
allowing known forms of curvature sensors to be placed at the top.
As another example, if a turnaround loop is used and it is desired
to measure curvature at the centre of a curved beam, then the beam
must be long enough to accommodate the turnaround.
Another disadvantage of the turnaround is that it must be held in
position to avoid changes in light intensity due to changes in
curvature within the plane of the turnaround, particularly if the
turnaround has a small radius of curvature which is producing light
losses substantially greater than those of a straight fiber. If the
turnaround is rigidly affixed to the substrate, this may produce
stresses on the fibers between the turnaround and the location of
the sensitized zone, which must also be rigidly attached to the
substrate in order to properly sense its curvature.
However, if the disadvantages of the turnaround can be overcome, it
has overwhelming advantages in terms of cost of manufacture, small
size, lack of complexity, and relatively low light loss.
The present invention provides an improved sensor means for sensing
curvature and displacement with minimum manufacturing cost and
minimum damage to the fiber.
An object of the present invention is to provide a sensor means
which minimizes residual light loss while optimizing sensitivity
and preserving the strength of the fiber.
A further object of the invention is to provide a sensor means
which allows maximum utilization of the portion of the linear range
exhibiting the greatest transmission of light through the
fiber.
A further object of the invention is to provide a sensor means that
allows achieving a given residual light loss and sensitivity over a
range of sensitized zone lengths.
A further object of the invention is to provide a sensor means
which allows placing the sensitized zone of the sensor near the
distal end of the sensor system.
A further object of the invention is to provide a sensor means
which maintains the orientation of the sensitized zone, once
treated.
SUMMARY OF THE INVENTION
This invention achieves these and other results by providing a
treatment means that minimizes residual loss and maximizes the
effect of mechanical curvature of the sensor on the transmission of
light through the fiber.
In one embodiment of the present invention, the sensor zone is made
up of alternately treated and untreated portions of the fiber.
There is opportunity for many light rays to refract toward the core
from untreated spaces between treated ones so that average
curvature may be sensed over a long length of the fiber without
introducing unnecessary residual loss. The treated portion or
portions may involve modification of the fiber for its entire
periphery or a part of the periphery. Treatment may include
abrasion, etching, repeated notching within the band, heating,
chemical removal, and others. Treatment is generally accompanied by
application of at least a thin layer of light absorbing
material.
In another embodiment of the present invention, treatment of
portions of the fiber mentioned above involves modification of only
a portion of the periphery, these portions being oriented on the
side of the fiber that is concave outward over a desired range of
curvatures. By varying the lengths of treated and untreated strips,
and the number of strips, and the extent of the periphery treated
by each strip, the size of the linear region of sensitivity for
concave bends can be increased, so that the sensor is operated near
its maximum throughput condition. This embodiment has the advantage
of allowing operation of the sensor within its highest linear
throughput range, and controlling the size and placement of the
centre of that range, while maintaining linearity of response to
bending and displacement. The method has the advantage of being
able to produce a sensor that has a minimum amount of periphery
treated, while still providing maximum sensitivity and throughput
over a desired range. This is important to maintaining the strength
of the fiber used, especially when glass fibers are being
treated.
In another embodiment of the present invention, the turnaround at
the distal end of the fiber sensor loop is treated to be sensitive
to bending, such that the treated portion has a minimal effect on
throughput of the turnaround, but the sensitivity to bending is the
same or improved over that of a sensitized zone placed on a
straight portion of the fiber. In this case, the net throughput of
the sensitized zone and the loop combined can be made greater than
for a sensor fiber in which the zone and loop are separated. The
sensitized turnaround loop makes it possible to sense curvature at
the free end of a structure, and has the added feature that if the
turnaround loop is heat-formed or constrained by a form or fixture,
it forms a plane that is always orthogonal to the plane of maximum
sensitivity of the sensor, so that it is easy to maintain
orientation of the maximum sensitivity plane during manufacture.
The structure also lends itself to easier manufacture, because the
turnaround forms the distal end of the fiber loop, and can easily
be inserted into a machine for heat treatment, embossing, sanding,
or other operations. This is not the case for a fiber which must be
treated at some arbitrary location along its axis, especially if it
must be inserted into an oven for heat treatment, without involving
the leads in the treatment. This embodiment can be used with
various forms of treatment, including treatment of various portions
or all of the circumference.
The invention may be defined as a fiber optic curvature and
displacement sensor comprising a fiber optic light guide having at
least one light emission surface extending, for part of the length
of the guide, in a direction selected from: a substantially
circumferential direction, and a substantially curved axial
direction when in a curved portion of a curved guide.
In one preferred embodiment, the light emission surface is in the
form of a plurality of circumferentially-oriented bands.
In another preferred embodiment, the light emission surface is in
the form of at least one ring around the circumference.
In a further preferred embodiment, in a fiber having a rectangular
cross-section, the light emission surface is on one of the sides of
the rectangle, or on opposed sides of the rectangle. Preferably the
rectangle is oblong, with the light emission surface or surfaces on
one or both longer sides, respectively.
Preferably the light emission surfaces are positioned to give the
most desirable orientation to the maximum sensitivity plane.
The fiber optic light guide may be in the form of a loop, the loop
having at least a substantial portion of the light emission surface
or surfaces therein. These latter surfaces preferably are in a
configuration selected from peripherally-oriented bands grouped on
an inside or concave portion of the curvature of the loop, and
axially-oriented bands substantially parallel to the plane of the
loop and substantially following the curvature of the loop.
In one preferred embodiment, from three to five
circumferentially-oriented bands are grouped together with the
bands each extending for about 60.degree. to about 90.degree. of
circumference. In the case of axially-oriented bands, preferably
from two to six curved bands are either coplanar or in parallel
planes. The invention includes sensors in which the fiber optic
light guide, or portion thereof, is in the form of a single fiber
which, serves both to illuminate the light emission surface or
surfaces, and to collect illumination which has passed the light
emission surface or surfaces, the illumination end and collection
end of the fiber being located in a single region removed from the
light emission surface or surfaces.
In a further preferred embodiment, the fiber is in the form of a
flat strip, having a generally rectangular cross-section having
spaced parallel opposed wide sides. The light emission surface or
surfaces are formed on one or both of the wide sides. In
particular, the light emission surface or surfaces are formed at a
bend in the fiber, or may include both axes. The bend may be in the
plane of the wide axis of the fiber, or in the plane normal to the
wide axis of the fiber. One example of a strip-type fiber is one
formed from a strip of polymer material, such as is sold under the
trade name Mylar.
Yet a further preferred cross-section can be a D-shaped
cross-section, with the light emission surface being on the flat
side of the D. Normally the fiber will be bent in the plane of the
flat side. Other cross-sections and arrangements can be
provided.
The invention includes, in a vehicle suspension system which
includes an electronic system for sensing displacement between a
vehicle body or frame and a vehicle wheel system and comprises a
suspension sensor, the improvement comprising a flexible beam and a
fiber optic curvature and displacement sensor mounted to the
flexible beam, the sensor comprising a fiber optic light guide
having at least one light emission surface extending, for part of
the length of the guide, in a direction selected from: a
substantially circumferential direction, and a substantially curved
axial direction when in a curved portion of a curved guide.
The invention also includes a method of sensing curvature and
displacement, of an elongate member, comprising attaching a fiber
optic light guide to the member, the light guide having a light
emission surface extending, for part of the length of the guide, in
a direction selected from: a substantially peripheral direction,
and a substantially curved axial direction when in a curved portion
of a curved guide, the light guide extending along the member;
injecting a light beam into one end of the light guide, detecting
the light beam at the other end of the light guide, measuring the
difference in the light beam between the one end and the other end,
indicating curvature or displacement, of the member.
Preferably the method includes selecting the light guide so as to
optimize sensing of curvatures over a range that includes, as a
substantial portion of the total substantially linear range sensed,
curvatures that produce increasing transmission of light with
increasing curvature.
Preferably the method includes attaching a plurality of fiber optic
light guides to the elongate member, each having a plurality of
light emission surfaces such that the planes of maximum sensitivity
of the guides are at different angles from each other so that at
least one guide maximally indicates curvature at a unique planar
inclination.
A preferred arrangement may utilize two to six fiber optic light
guides oriented in different directions at predetermined angles
relative to each other.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be readily understood by the following
description of certain embodiments, by way of example, in
conjunction with the accompanying drawings, in which:
FIG. 1 is a diagrammatic illustration of a bending sensor
apparatus, with the sensor shown cemented to a bending beam;
FIG. 2 is a cross-section through an optical fiber in accordance
with the present invention;
FIG. 3 is a cross-section similar to that of FIG. 2, illustrating a
modification thereof;
FIG. 4 is a perspective view, on a large scale, of part of a light
guide showing a form of surface treatment;
FIG. 5 is a side elevation of the light guide of FIG. 4;
FIG. 6 is a transverse cross-section of the sensor as on line X--X
in FIG. 5;
FIG. 7 illustrates an alternate form of surface treatment of the
light guide;
FIG. 8 is a side elevation of the light guide of FIG. 7;
FIG. 9 is a transverse cross-section of the sensor as on line Y--Y
of FIG. 8;
FIG. 10 is a transverse cross-section of a sensor showing the axes
of maximum and minimum sensitivity to bends;
FIG. 11 is a perspective view of a sensor employing a non emitting
reference fiber paired with an emitting, sensing, fiber;
FIG. 12 is a perspective view of a triple sensor for detecting the
three-dimensional bend vector;
FIG. 13 is a cross-sectional view of the triple sensor on the line
Z--Z of FIG. 12, showing the 120.degree. arrangement of emitting
sections;
FIG. 14 is a schematic diagram showing light paths and electronic
circuitry;
FIG. 15 shows an alternative form of the light guide return
path;
FIG. 16 shows an alternate application of the sensor, to
measurement of position;
FIG. 17, 18 and 19 are graphs of the percentage changes in
transmission of light guides treated in various ways to sense
bending;
FIGS. 20 and 21 are graphs of the percentage changes in
transmission for an opposed sensor pair made from two light
guides;
FIG. 22A is a side view of a fiber with alternating bands of
treated and untreated material;
FIG. 22B is a cross section through one of the untreated portions
of the fiber in FIG. 22A;
FIG. 22C is a cross section through a treated portion of the fiber
in FIG. 22A;
FIG. 23A is a side view of another type of treatment, showing
alternating bands of treated and untreated material, the treatment
not encompassing the entire circumference of the fiber;
FIG. 23B is a cross section through one of the untreated portions
of the fiber in FIG. 23A;
FIG. 23C is a cross section through one of the treated portions of
the fiber in FIG. 23A;
FIG. 23D is a cross section through a fiber as in FIG. 23A, but
treated in an alternate manner;
FIG. 24A is a longitudinal cross section of a straight section of
fiber with three emission surfaces as in FIG. 23A, showing ranges
of light rays subtended by emission surfaces;
FIG. 24B is a longitudinal cross section of a downward curving
fiber as in FIG. 24A;
FIG. 24C is a longitudinal cross section of a upwardly curving
fiber as in FIG. 24A;
FIG. 25 shows a family of curves showing light loss through a fiber
treated with successively more emission surfaces, as it is bent
over a wide range of curvatures;
FIG. 26 shows a more detailed graph of a fiber sensor treated as in
FIG. 25, to have a linear region centred on zero curvature as well
as data from another type of sensor;
FIG. 27 shows a sensor system including a loop of fiber used to
return light to the optoelectronic measuring system. The loop is
treated to act as a sensor and is mounted to a bending beam to
measure curvature near the end of the beam;
FIG. 28A is a longitudinal section of the loop in FIG. 27. The top
portion of part of the cladding in the loop has been removed to
allow light to escape in two patches on a side of the fiber. The
patches are located on curved portions of the loop;
FIG. 28B is a plan view of the fiber in FIG. 28A;
FIG. 28C is a cross section through one of the treated portions of
the fiber in FIG. 28A;
FIG. 28D is a cross section through one of the untreated portions
of the fiber in FIG. 28A;
FIG. 28E is a cross section through a treated portion of another
embodiment like the fiber shown in FIG. 28A except that both sides
of the loop have been treated;
FIG. 28F is a cross section through a treated portion of another
embodiment like the fiber shown in FIG. 28A except that the treated
portion is located on the inner portion of the loop and is in a
substantially circumferential orientation;
FIG. 28G is a plan view as in FIG. 28B, except that the emission
surface is continuous;
FIG. 29A shows a plan view of a turnaround loop as in the
embodiments portrayed in FIGS. 28A through 28G. Two rays which
originate as rays in the plane of the loop are shown travelling
through the fiber, remaining in the said plane;
FIG. 29B shows a cross section through the fiber just before it
begins to bend, showing the two rays still in the said plane;
FIG. 29C shows a cross section through the fiber at the centre of
the loop, showing the position of the two rays, which are still in
the said plane;
FIG. 30A shows the same loop as in FIG. 29A but with a ray that is
parallel to the plane of the loop but displaced vertically;
FIG. 30B shows a section through the fiber just before it begins to
bend, showing the position of the ray above the plane of the
loop;
FIG. 30C shows a section through the fiber, containing the ray,
showing deflection of the ray downward as it refracts from the
outer wall of the loop;
FIG. 30D shows another section of the fiber, containing the ray,
showing deflection of the ray again downward, at such an angle that
it is lost from the fiber at the outer wall of the loop;
FIG. 30E shows sensor loss due to curvature for a fiber loop with
two emission surfaces, and for a similarly treated straight section
of fiber;
FIG. 31A is a plan view of a loop treated to sense curvature of a
diaphragm, attached along the surface of a diaphragm;
FIG. 31B is an elevation view of the sensor of FIG. 31A;
FIG. 32A is a plan view of a sensor as in 31A, except the loop is
supported in a curve from above the diaphragm and touches the
diaphragm at a point, so as to sense displacement of the
diaphragm;
FIG. 32B is an elevation view of the sensor system of FIG. 32A;
FIG. 33A is a plan view of a loop treated to sense curvature,
attached at its apex to a rotating drum or shaft, for the purpose
of indicating position of the shaft according to a varying
curvature imposed on the loop;
FIG. 33B is an elevation view of the sensor structure of FIG.
33A;
FIG. 34 is a transparent view of a joystick input device containing
loops of fiber treated and arranged to sense displacement of the
flexible joystick handle in two degrees of freedom;
FIG. 35A is a plan view of a loop treated to sense curvature,
mounted as a cantilever beam for sensing vibration and
acceleration;
FIG. 35B is an elevation view of the sensor of FIG. 35A;
FIG. 36 is a side view of a spring and fiber loop system designed
to translate large displacements of a body into relatively smaller
displacements of the end of the loop;
FIG. 37 is a schematic diagram showing light paths and electronic
circuitry;
FIG. 38A is a perspective view of one form of a rectangular
cross-section strip form of sensor;
FIG. 38B illustrates the sensing deflection of the sensor of FIG.
38A;
FIG. 39A is a perspective view of an alternative form of a
rectangular cross-section strip;
FIG. 39B illustrates the sensing deflection of the sensor in FIG.
39A;
FIG. 39C is a side view of the sensor in FIG. 39A;
FIG. 40A is a perspective view of a sensor having a D-shaped
cross-section;
FIG. 40B is a cross-section of the sensor of FIG. 40A;
FIG. 41 is a plan view of an alternative form of a rectangular
cross-section strip;
FIG. 41' is a graph of light transmission through a loop sensor
made of light guide with a rectangular cross-section; and
FIG. 42 illustrates a means of attachment useful in sensing angular
displacement.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIGS. 1 to 21, illustrate various forms of sensors, associated
electronic circuitry and output curves as claimed in
above-mentioned application Ser. No. 07/915,283.
FIG. 1 illustrates a bending sensor 10 mounted with adhesive on
bent beam 11. In the example, light is conveyed from a
photo-emitter 12 through a plastic or other optical fiber light
guide 13 to the sensor portion 10, and thence through guide 14 to a
photo-detector 15. The light guide near the sensor region 10 has
had its outer protective jacket removed, and the light conducting
core exposed along a strip on the surface; portions 13 and 14
leading to the sensor region may have the jacket in place. The
sensing portion 10 is adapted to sense bending. The photo-emitter
12 and photo-detector 15 are part of an electronic measuring system
16 and display 17.
FIG. 2 illustrates, in cross-section a conventional optical fiber
wave guide 18 having a light conducting fiber 19 and a cladding 20.
Normally there will be a buffer layer and a coating layer also. The
cladding is removed locally, at 21, extending in a band along the
fiber 19, to form a light emitting surface 22. The band can be
formed by deliberately removing cladding as by abrasion, melting,
etc. or by displacement as by pressure or rubbing on the fiber, for
example by a heated tool, depending upon the particular form of
fiber.
FIG. 3 illustrates a modification of the arrangement of FIG. 2, in
which the light emitting surface band 21 is covered with a light
absorbent material 20a. Typical materials for the coating 20a are
graphite filled epoxy resin, dye-filled resins and similar
materials. The use of the coating 20a prevents emitted light
interfering with any other instrument or structure and also
prevents any back reflection into the fiber, which would affect the
measurements. The additional coating 20a can be applied only over
the band 21, but more commonly is applied around the entire
fiber.
FIGS. 4, 5 and 6 illustrate one example of a fiber 19 with the
emitting surface textured. Serration 23 have been created on one
side of the fiber, as by pressing it onto the surface of a file.
Similar serration can be created by heat forming and moulding. Both
plastic and glass optical fibers can be so formed. Heat forming can
be accomplished by pressing the fiber slightly onto a heated metal
surface which can be serrated, corrugated, or otherwise formed. The
angle of the serration can vary. It is not necessary to first
remove the cladding of the fiber as this will be displaced. After
treatment, a sensor portion emits some light along the length while
transmitting a portion of any light within it to either end.
FIGS. 7, 8 and 9 illustrate another form of serration of a fiber
19. In this example the wedge-shaped serration 23 of the sensing
portion 10 are separated by small spaces 24. The exact shape of the
serration can vary considerably. Diamond-shaped serration have also
been successfully used. These are formed by pressing the fiber
against a file with a pattern of intersecting serrations.
Alternatively, one side of the fiber can be abraded by sanding,
sand-blasting, etching, or other means of removing or changing the
cladding layer.
FIG. 10 illustrates the axes of maximum and minimum sensitivity,
and the direction of signal change of a bending sensor. In this
cross-section, the light emitting surface band is 22 at the top of
the sensing section of the fiber. Bends within the vertical plane
containing A-A produce the maximum change in transmission of light
through the fiber. Thus A--A is called the axis of maximum
sensitivity. For bends concave upward, the transmission increases.
For bends concave downward, the transmission decreases. The minimum
change in transmission occurs for bends in the horizontal plane
containing B--B. Bends in this plane produce negligible change in
transmitted light, so B--B is called the axis of minimum
sensitivity. Intermediate response occurs for bends off the major
axes, such as at C--C. This intermediate response is a cosine
function of the angle between the plane of maximum sensitivity and
the plane of the angle of the bend. In the figure, + and - signs
have been placed to indicate increases and decreases in transmitted
light relative to the transmission of the fiber when it is
straight. The surface band 22 can be just bare fiber, as in FIG. 2,
or textured, as, for example, in FIGS. 4 to 9.
FIG. 11 illustrates a sensor including a paired reference fiber.
Fiber 19 has a sensing portion 10. Fiber 25 has no sensing portion.
The pair are used in dual detection methods, where all measurements
are referenced to the transmission through fiber 25. Because fiber
25 is arranged mechanically in the same way as fiber 19, most
errors are eliminated from the measurement, by virtue of the fact
that untreated fibers show little change in transmission for small
bend angles (roughly less than 20.degree., whereas formed fibers
are optimized for response to bending).
FIG. 12 illustrates three fibers, arranged to form a sensing system
capable of detecting the three-dimensional vector describing the
applied bend. Each fiber has a light emission portion 10, just bare
fiber or formed with serration or abrasions. The sensing portions
are arranged so that the axes of maximum sensitivity are at
120.degree. to each other. FIG. 13 shows this relationship more
clearly. Solving simultaneous equations for the magnitude and sign
of the transmissions of the fibers will yield the three components
of the bend vector of an element, such as a beam, to which the
sensor has been affixed. Alternate arrangements of the fibers are
possible, such as having the sensing portions facing at different
angles, triangular rather than flat bundles, or having the fibers
separated from one another. In the examples illustrated in FIGS.
11, 12 and 13, the fibers are conventional in that they are
composed of the main fiber, of glass, plastic or other, with a
cladding layer. The cladding layer is locally removed at the sensor
positions 10 as mentioned above, by various means.
FIG. 14 illustrates one simple example of electronic circuitry that
can be used to measure the transmission of light through a paired
sensor element such as that shown in FIG. 11. In FIG. 14, fiber 19
(shown coiled to indicate arbitrary placement and length of the
guide conveying light to and from the sensing portion) has the
light emitting strip at 10. Fiber 25, which is otherwise the same
as fiber 19, has no sensing portion at position 26, which
represents a section of the fiber in close proximity to sensor
section 10. Both fibers are illuminated by photoemitters E1 and E2,
which are light emitting diodes. Photodetectors D1 and D2, which
receive light from the fibers, are PIN photodiodes, back-biased
with 12 Volts to enhance the speed of their response to light
energy. U1 and U2 are high input impedance operational amplifiers
arranged as transimpedance amplifiers, converting light energy
linearly into voltages fed to the inputs of U3, which is an
operational amplifier connected as a differential amplifier with a
gain of 10. The gain of amplifier U2 can be varied with R1 so that
for a straight fiber, the inputs to U3 are equal. In this
condition, the optoelectronic circuit is analogous to a two-armed
bridge such as is used to make strain-gauge measurements. Errors
due to degradations in the fibers, connector variations,
temperature fluctuations, and the like tend to cancel before
reaching the output of U3. The output of U3 is a voltage which
aries with bending at the band portion 10. The output voltage can
be further amplified and sent to a display unit or used to control
various parameters, such as actuators designed to minimize the
angle of bend.
Many variations of the circuitry are possible, including variations
with much greater immunity to error sources. One such variation
would use the same light source and detector, separating the
signals by chopping them at different frequencies and employing
synchronous detection. Another variation is to replace U3 in FIG.
14 with a divider circuit, so that the sensor signal is divided
arithmetically by the reference signal.
Another variation uses one photoemitter, for example E1 in FIG. 14,
to illuminate the sensor fiber, such as 19 in FIG. 14, and also to
illuminate a reference fiber such as 25 in FIG. 14. This last
variation may be further enhanced by eliminating U3, using U2 and
another amplifier to control the light out of 25 to constant value,
and reading relative light transmission through 19 directly from
the output 12.
FIG. 15 illustrates a variation of the sensor design which
eliminates loops of light guide past the sensing portion. Light
from a photo-emitter enters the system throughguide 19 and passes
through the sensing portion 10. Another sensing portion 27 faces
sensing portion 28 on guide 29. An opaque cap 30 filled with
optically clear adhesive 31, covers this junction area. Guide 29
carries light back to a photo-detector. The junction region at 27
and 28 is held rigidly by the cap and adhesive so that it does not
respond to bending. This arrangement allows the use of parallel
fibers without return loops, which can be an advantage when
embedding the sensors in long, narrow structures.
Alternatively, the fibers 27 and 28 can be non-serrated, and
inserted part way into the opaque cap 30, in this variation, the
cap is preferably filled with translucent white polymer such as
epoxy mixed with white pigment. Diffusion of light through the
polymer accounts for transmission of light from fiber 27 to fiber
28.
Alternatively, a mirror can be used to reflect light from the
distal end of the sensor fiber back into a return fiber. In another
alternative, a directional coupler can be used with a single sensor
fiber which has a mirror mounted at its distal end. The coupler is
used to pass light to the proximal end of the fiber from the
emitter, and to direct reflected light emanating from the proximal
end of the fiber, into the detector. Typically, the directional
coupler would be placed between the emitter, the detector, and the
proximal end of the fiber.
FIG. 16 illustrates the application of the bending sensor to the
measurement of displacement. Vertical displacement 32 between
blocks 33 and 34 (representing, for example, moving parts of a
mechanical system), is measured by a bend-sensing region 10 in
guide 19, cemented into flexible beam 35, which is attached to the
two blocks. Similar fixturing, with or without a flexible beam,
would enable the measurement of angles, such as pedal position,
over a large range.
The advantages of the sensing devices described above are
illustrated in FIGS. 17, 18 and 19. For these figures the sensors
were fabricated from methyl methacrylate optical fiber, 1 mm in
diameter. The black plastic jacket was left on the fiber except for
a 50 mm section near the sensing portion, where the black plastic
was removed.
FIG. 17 illustrates, by way of example, the percentage change in
transmission, that is the amount of light transmitted compared to
100 percent for a straight fiber, of a 1 mm diameter plastic fiber
formed over a 25 mm length with serration and affixed with epoxy
adhesive to a plastic beam. Weights were placed on the beam, which
was clamped at one end, to produce the angular deflections shown on
the horizontal axis of the graph. Two outputs of the sensor are
shown. One is from the sensor when the beam is mounted to bend
along the axis of maximum sensitivity (serration pointing up); the
other is the output when the beam is mounted to bend along the axis
of minimum sensitivity (serration pointing horizontally). There is
virtually no sensitivity to bending along the axis of minimum
sensitivity. The sensor response in the axis of maximum sensitivity
is essentially linear with respect to angular deflection,
increasing for upward bends and decreasing for downward bends.
The graph illustrates the sensitivity obtainable with very simple
electronics (for example, as shown in FIG. 14). One could measure
beam deflection with a strain gauge on the top of the beam. The
strain gauge would be responding to elongation of the top surface
of the beam rather than curvature of the top surface. The beam is
undergoing an elongation of approximately 120 microstrain (microns
per meter) along its top for a 3.degree. deflection, indicating
that the optical sensor is achieving a resolution of better than 12
microstrain (120 microstrain over 10 data points) for this
experiment. Other sensors have been constructed which are capable
of resolving less than 1 microstrain, which is the approximate
lower sensitivity limit for strain gauges. Because it is possible
to measure curvature with a bending sensor on the neutral axis of
the beam, where there is, by definition, no strain, the lower limit
for strain sensitivity is zero.
FIG. 18 illustrates, by way of example, the response of a fiber
light guide having serration over a 25 mm length compared to the
response of an unformed fiber. The fibers were clamped near one end
of the sensitive region and moved so as to bend in a loose curve of
the total angle shown on the horizontal axis of the graph. The
fiber with the sensing portion was moved in the plane of maximum
sensitivity. The fiber demonstrates a large range of linear
response; approximately .+-.20.degree.. When moved similarly, the
unformed fiber, an example of a microbend sensor, showed virtually
no change with angle, except for angles over 20 degrees, where the
response is approximately 20 times less than the formed fiber.
FIG. 19 illustrates, by way of example, the response of three
fibers having the textured surface formed in different ways. The
test setup is the same as for FIG. 17. The first fiber has
serration over a 25 mm length. The second is formed by abrading
slightly with fine sandpaper around the circumference of the fiber
over a 25 mm length. The third is formed by abrading slightly with
the same sandpaper over a 25 mm length, but only on one side of the
fiber, similar in overall shape to the region of serration of the
first fiber. The two fibers having the textured surface on one side
show good preservation of direction of the bend in their responses.
The fiber having the textured surface on its entire surface shows
no preservation of the direction of bend. It would have to be bent
when mounted on a structure. The serrated fiber shows linearity of
response over a wider range than the fiber sanded on one side.
FIG. 20 illustrates, by way of example, the response of a sensor
pair mounted with epoxy adhesive inside a small piece of plastic
tubing. Each sensor was formed by removing cladding along a strip
on the side of a silica fiber core, 200 microns in diameter. The
strip was 10 mm long. The two strips were coated with
graphite-filled epoxy and arranged to face in opposite directions,
so that response to bends was of opposite polarity but
approximately equal magnitude for each sensor. The "percentage
change in transmission" shown in FIG. 20 refers to the difference
in the two electrical signals resulting from light transmission
changes in the two sensors, as a percentage of the electrical
signal from one sensor at rest. This paired method of measurement
is one means of greatly decreasing the effects of common-mode
emitter and detector drift. In this example, the tubing was first
bent, by means of a micrometer drive, upward in the plane of
maximum sensitivity (positive horizontal graph axis), away from its
rest position (zero on horizontal axis). Next it was moved back
toward its rest position. These steps were repeated for downward
bends. The data points in FIG. 20 illustrate the excellent
linearity of the system, and the low hysteresis ("away from zero"
and "toward zero" points are nearly coincident). A straight line
("Linear Fit") has been drawn on FIG. 20 to illustrate the
excellent linearity of response.
FIG. 21 shows the same data points as FIG. 20, over a small range
of bends near the origin.
FIGS. 22 to 42 illustrate further embodiments in accordance with
the present invention.
FIG. 22A illustrates a fiber 38 with alternating circumferential
bands of treated material 39 and untreated material 40. The figure
shows the layers of the fiber out to the level of the cladding 43.
The bands are formed by modifying the core and or cladding so that
light is lost from the core. The bands can be formed by
deliberately removing cladding as by abrasion, melting, etc. or by
displacement as by pressure or rubbing on the fiber, for example by
a heated tool, depending on the particular form of fiber. The
treated bands are covered over with light absorbing material 41
which provides mechanical strength and environmental protection
where cladding has been modified, but whose primary purpose is to
absorb all light exiting the fiber from the core 42, which is shown
in FIGS. 22B and 22C. Without this layer, the fiber can still act
as a light guide because air surrounding it will have an index of
refraction lower than that of the core. The fiber may actually
transmit more light if the cladding is removed and no light
absorbing coating is applied, thus preventing it from properly
measuring curvature. In any case, without an absorbing coating the
fiber will exhibit a nonlinear response that varies over time,
especially if reflective materials, liquids, and dirt are present.
With absorbing coating the response of the fiber is very constant
over time and is unaffected by environmental factors. The absorbing
coating may consist of carbon-filled epoxy, dyed elastomer,
carbon-filled hot glue, or any other substance that permits light
to exit the fiber but prevents it from re-entering in any
substantial quantity. The light absorbing coating may serve other
functions as well, such as protection of the fiber against
environmental contamination. It may be applied only to the sensor
portion of the fiber or may be incorporated into a part, such as a
rubber or graphite/epoxy part, into which the sensor is
embedded.
FIG. 22B is a cross section of fiber 28 through X--X where no
treatment has been performed. Cladding layer 43 covers core
material 42. Little light can escape from this section because the
index of refraction of the cladding is less than that of the core.
Excess absorbing material (not shown in this figure) may for
convenience or for structural reasons cover the cladding in any of
the untreated portions of the fiber.
FIG. 22C illustrates a cross section of the fiber 38 through Y--Y,
a treated portion. Where the cladding has been removed or modified,
it is replaced by absorbent material 41, which covers the core 42
and any remaining cladding (not shown in this figure).
FIG. 23A illustrates a fiber 44 with cladding 43, that has been
treated as in FIG. 2A except that the treated bands 39 cover a
partial extent of the circumference.
FIG. 23B shows a cross section through Z--Z, an untreated portion
of fiber 44. This untreated portion is identical to that in FIG. 2B
and includes a core 42 and a cladding 43.
FIG. 23C shows a cross section through P--P, a treated portion of
fiber 44. The cladding has been modified or removed over part of
the circumference and back filled with light absorbing material
41.
FIG. 23D is a cross section through a fiber treated like the fiber
in FIG. 23A but with both upper and lower surfaces treated to emit
and absorb light. Although it is treated on both sides, it too will
be maximally responsive to bends in the plane of maximum
sensitivity passing through the centres of the two treated arcs.
Its response to bends outside this plane will be a cosine function
of the angle between the plane of maximum sensitivity and the other
plane.
This cosine law is what allows the use of multiple sensor fibers
with different angles of planes of maximum sensitivity to that
three dimensional bands can be resolved by a set of sensors. It
applies to any of these sensors except ones that include bands or
rings which extend the entire circumference. The best cosine effect
happens if up to half the circumference is treated to form a band.
Otherwise, there is a "DC offset" added to all the cosine responses
up to the point that the signal does not change with different
plane angles--this happens when the bands are completely
circumferential in extent.
FIG. 24A shows in longitudinal cross section a cone of light with
an angle Ct that represents the solid angle of light that meets the
internal refraction criteria for a fiber such as fiber 44 in FIG.
23A. This angle is called the acceptance angle of the fiber. The
size of Ct is determined by the relationship between the indices of
refraction of the core and cladding.
The acceptance angle can be related to the indices of the core and
cladding through
where the argument of the inverse sin function is called the
numerical aperture of the fiber, which is determined from the index
n.sub.1 and n.sub.2 of the core and cladding respectively. Rays
leaving representative point 46 with angles included in Ct will be
refracted back into the core wherever they strike the cladding, and
continue propagating down the fiber. Rays within angular ranges C1,
C2, and C3 will strike the treated portions 41 and be lost.
FIG. 24B shows angular ranges C4 and C5 within total cone angle Ct
for the fiber 44 in a bent state. Because of the bend, the treated
portions subtend smaller angles of light, so that C4<C1,
C5<C2, and there is no C6 corresponding to C3 in FIG. 23D. This
is the mechanism which produces an increase in light level as the
fiber is bent to make the treated portions become more concave.
Conversely, as it is bent in the other direction, the angles
subtended by the treated portions become larger and represent a
larger portion of the total acceptance angle Ct. This causes a loss
of light compared to a straight fiber.
It is possible to increase the range of curvatures over which the
fiber will have a substantial change in throughput for bends making
the treated portions more concave. This can be done by adjusting
the axial lengths of the individual light emission surfaces and the
length of the entire treated zone. FIG. 25 shows the results of
tests of a fiber treated with successively more emission surfaces
along an axial line of the surface of the fiber. Each surface was 3
mm long, and spaced from the others axially by 3 mm. Data are shown
for 2, 4, 6, and 8 emission surfaces. The fiber was held over
various round mandrels to produce the curvatures shown. Curvatures
include positive (emission surfaces concave) and negative (emission
surfaces convex) values. From the family of curves in FIG. 25 it is
evident that when there are fewer surfaces, the positive curvatures
tend to produce changes in light that are not linear with
curvature, but the negative curvatures tend to produce linear
changes. This effect is very evident in the 2.times.3 mm curve. As
more surfaces are added, the linear portion of the curve moves
toward the positive curvature region, as in the 8.times.3 mm curve.
Intermediate values (as in the 6.times.3 mm curve) can be chosen to
place the linear range approximately intermediate between positive
and negative curvatures.
FIG. 26 shows a more detailed graph of a fiber sensor treated as in
FIG. 25 to have a linear region centred on zero curvature, as well
as data from another type of sensor. The "centred" data are shown
in the curve for "Treatment 1." The data for "Treatment 2" are from
a similar fiber that was treated over its entire circumference so
that it has a negative sloped response for positive curvatures.
Together, FIGS. 25 and 26 show that by applying various methods of
treatment, it is possible to place the linear ranges of the fiber
response curves so that they reflect various ranges of curvature,
and to change the slope of curvature over a wide range.
The various curves shown in FIG. 25 are explained in the following
way:
For a fiber bent so as to make its emission surfaces more convex,
(negative curvature in FIG. 25), the surfaces continue to intercept
more rays the farther the fiber is bent. The fiber may be bent
substantially in this direction with an ever increasing loss of
light. However, the fiber will have a nonlinear response for very
large negative curvatures (as seen in FIG. 25, 8.times.3 mm),
because of the failure of the emission zones to intercept
additional rays. As the fiber is bent the positive way, fewer rays
are intercepted (see FIG. 24C) as the bands take on an increasingly
concave form. However, for a short emission surface, the surface is
soon substantially out of the path of the rays. There is then no
further loss of light, because it is predominantly reflected from
the side of the fiber away from the emission surfaces, and does not
interact with them. The interaction is least when there are few
emission surfaces, as evidenced in FIG. 25, 2.times.3 mm and
4.times.3 mm. The 2-surface fiber (and a 1.times.3 mm surface fiber
which is not shown) even shows an increased loss for the largest
curvature tested, probably due to microbending losses predominating
over emission surface losses.
In one embodiment, the present invention utilizes such a large
linear region for positive curvatures to increase the net
throughput of the sensor system. By spacing the emission surfaces,
it is possible to both increase the sensitivity and size of the
linear region for positive curvatures and to minimize the residual
light loss because a substantial number of rays can still pass the
sensor region without attenuation. The extended structure permits
distant bands to intercept light rays that are nearly parallel to
the fiber axis, even when the curvature change is minimal. This
leads to a high sensitivity with minimum residual light loss. If
one attempts to achieve high sensitivity for positive curvatures by
lengthening a single emission surface, a limit is reached where the
increase in residual light loss exceeds any gains in sensitivity,
even though linearity is maintained.
For 1 mm plastic fibers, typical light emission surfaces for
efficient sensors are approximately 2 to 10 millimetres in length,
spaced by 2 to 10 millimetres. The overall length depends on the
desired range but is typically up to 50 cm. Surfaces for smaller
glass or plastic fibers are typically smaller. Notches may be used,
but will not achieve the same sensitivity as uniform surfaces with
smaller surface texture dimensions. This is probably because
notches perform an emission function but also tend to scatter light
back into the fiber. This is particularly true if they are not
covered with a light absorbing layer. Notches have the further
disadvantage of weakening the fiber.
If the emission surfaces occupy the total circumference of the
fiber as in FIG. 22A, there is no increase in throughput for bends
of the fiber in any plane. All of the sensing is done through
decreases in light level. Nevertheless, spaced emission surfaces
are still an advantage for many sensors, as they can be used to
sense average curvature over a greater axial length of the fiber.
This can eliminate or reduce undesirable effects from large local
changes in curvature, for instance due to the presence of a foreign
body under the fiber.
Ordinarily, a loop is used to return the light signal to the
optoelectronic measuring system. Often, space is limited so that
the loop must be formed in a tight curve such that substantial
amount of light is lost from its outer convex surface. When
combined with the residual light loss of a sensor elsewhere on the
fiber, the resulting total light loss may be excessive. The "loop
sensor embodiment" is designed to reduce this total loss by placing
the emission surfaces in a novel manner.
In FIG. 27 is shown a sensor system designed to have maximum
throughput even though it includes a loop 48 at the end that may be
in a tight curve that loses light at its convex outer surface. This
figure includes the same components as in FIG. 1 except that the
treated section 10 is on the loop 48 instead of on a straight
section of the fiber. The sensor is designed to measure curvature
of the substrate to which the loop is attached. In FIG. 27, the
sensor would be used to measure curvature at the end of the beam
11.
A detailed drawing of one embodiment of the sensor of FIG. 27 is
shown in FIGS. 28A through 28F. In FIG. 28A, the loop is shown in
longitudinal section. Emission surfaces 49 have been formed in one
surface of the fiber by removing cladding 51 from core 50 and
replacing it with light absorbing material 41. In FIG. 28B, the
emission surfaces 49 are shown in plan view. Cross sections in FIG.
28C and FIG. 28D show sections through treated (D-D) and untreated
(E--E) portions of the fiber respectively. The number of emission
surfaces is representative only. Larger loops could contain more
emission surfaces. Emission surfaces may be formed on surfaces
above or below the plane of the loop, or on an inside or concave
portion of the curvature of the loop. FIG. 28E is a cross section
illustrating emission surfaces on both the upper and lower surfaces
of the loop. FIG. 28F is a cross section illustrating an emission
surface on the inner concave portion of the loop. FIG. 28G shows
another variation of loop sensor wherein the emission surface 49
covers virtually all of the upper surface of the loop.
FIG. 29A shows the core 30 of the loop of FIGS. 28A through 28D.
Rays h1 and h2 are in the plane of the loop. They meet the outer
convex surface of the loop and are refracted around the loop to
continue on through the fiber. For rays substantially in this
plane, there will be little change in direction out of the plane as
they traverse the loop.
FIG. 30B shows a ray v1 that is substantially out of the plane of
the loop but parallel to it. The ray is shown in FIG. 30B, a
vertical (perpendicular to the page) cross section through A--A,
just before the ray enters the curve of the loop. The ray impinges
on the outer convex surface of the loop. FIG. 30C shows a vertical
section through the core containing the ray v1 after it refracts
from this first collision. Because it is substantially above the
plane of the loop, it is deflected downward by the curve of the
fiber and collides a second time with the wall of the core. FIG.
30D shows a vertical cross section through C--C, a plane containing
the ray v1 after its second collision. It is now travelling even
more downward and impacts the outer curve of the loop at an angle
such that it cannot be refracted back into the fiber, but travels
through the cladding (not shown) and is lost.
Thus, rays that are travelling out of the plane of the loop but
parallel to it will be deflected vertically as they travel through
the loop. Rays above the plane will be deflected downward. Rays
below the plane will be deflected upward. If they are sufficiently
above or below the plane, they end up being lost because they
strike the core/cladding boundary at too small an angle of
incidence to the normal due to the quasi-spiral reflections
indicated in FIGS. 30A through 30D.
We have seen above that there is a vertical impetus imparted to
rays that travel through the loop without being lost at the outer
convex surface of the loop. For convenience, these rays will be
called "survivor" rays. They are distinct from "doomed" rays
described FIGS. 30A through 30D that will either impinge directly
on the emission surfaces early in their travel through the loop or
be lost to the outer surface through excessive deflection. This
change in elevation causes them to interact more or less with
emission surfaces near the top or bottom of the loop. Thus, as the
loop is curved out of its plane by an external stimulus, the
emission surfaces interact with the survivor rays and cause the
throughput to vary with curvature in much the same way as it does
if the emission surface is located in a straight portion of the
fiber. By adjusting the length, spacing, and circumferential extent
of the emission surfaces, it is possible to change the throughput
so that it is linear with curvature and to adjust the midpoint of
the linear range so that it includes the zero curvature point.
With the sensor on the loop, the emission surfaces can be adjusted
so that a large proportion of the survivor rays interact with the
emission surfaces. Those survivor rays that do not get absorbed by
the treated emission surfaces for certain curvatures are by
definition the rays that get through the loop. This leads to a high
sensitivity of survivor rays to curvature. Survivor rays are made
up predominantly of rays that enter the loop in planes near the
horizontal (in the paper) plane of the loop and that have small
vertical components. By contrast, "doomed" rays arrive in
predominantly vertical planes with larger vertical components. If
instead of placing the emission surfaces on the curved loop, we
placed them on the relatively straight fiber nearby, rays
containing most of the sensor information would be in the vertical
plane as they enter the loop and thus would be doomed rays. This
would result in the light exiting the loop being made up mostly of
rays that have not been modulated by the sensor. This is equivalent
to reducing the sensitivity of the sensor. The same argument holds
for a sensor downstream of the loop. The light leaving an untreated
loop has been stripped of most of its vertical modes, so that a
sensor placed in this light stream will be modulating a minority
portion of the light passing through it. However, it is possible to
form a useful loop sensor wherein the emission surfaces extend
somewhat beyond the loop, as long as they are substantially on the
loop.
Although the loop sensor embodiment has advantages of increased
sensitivity and lower residual light loss for many arrangements of
emission surfaces, this should not be taken to be its only
advantage. Even in the absence of sensitivity and loss advantages,
there are compelling reasons to form a sensor on the loop. These
include sensing at the end of a structure, reduction of fiber
length, ease of mounting, reduction of stresses on the fiber
inherent in mounting a sensor portion and a loop portion separately
with free fiber in between, ease of manufacture, preservation of
orientation, and simplicity. In addition, loop sensors may be made
to be relatively free from responses to bends within the plane of
the loop.
The upper curve in FIG. 30E shows the output of a loop sensor made
by forming two axially oriented emission surfaces on the surface of
a 7 mm diameter fiber loop of 1 mm diameter plastic fiber. The
lower curve shows the sensor output when the same patches are
formed on a straight section of fiber 2 cm from a 7 mm diameter
untreated loop. For this arrangement of emission surfaces, the
output of the loop sensor is superior to the output of a sensor
formed apart from the loop.
FIG. 31A shows another embodiment of the loop sensor, wherein a
fiber 51, treated at the loop 52 to sense bending of the plane of
the loop, is attached to a flexible diaphragm 53. These same parts
are shown in FIG. 31B. As the curvature of the diaphragm 53
changes, the output of the sensor changes. This sensor structure
could be used to measure pressure or to form a membrane-type
keyboard, or to perform many other tasks wherein a diaphragm
undergoes changes in curvature.
FIGS. 32A and 32B show a sensor and diaphragm as in FIGS. 31A and
31B, wherein the fiber 51 is held in a fixed support block 57 so
that at least the sensor portion of the fiber changes curvature
according to the displacement of the diaphragm 53 at the point of
attachment or contact 62.
FIG. 33A shows a loop sensor wherein fiber 51 is treated at
multiple portions 54 to be sensitive to bending of the plane of the
loop. At the apex of the loop, it is attached to a turning shaft
55. FIG. 33B shows an elevation of the same sensor structure,
wherein it can be seen that the turning shaft can be turned over a
limited angular range clockwise or counterclockwise about pivot 56
and that the loop is attached to an anchor point 57. As the loop
winds around the shaft, its curvature increases, changing the
throughput of light.
FIG. 34 represents a joystick device wherein two loop sensors 58
and 59 are embedded in a vertical flexible shaft 60, attached to a
base 61. The loops have planes of maximum sensitivity at right
angles to each other, centred about the centre of the shaft. The
outputs of the fiber sensors represent orthogonal components of the
curvature of the shaft. In a miniaturized form, this sensor could
be a keyboard-mounted input device for a computer.
FIGS. 35A and 35B portray a loop sensor consisting of fiber 51 held
by fixed support 57, with a sensor zone 52 on the loop. The
structure to the right of the support 57 forms a cantilever beam.
It is used to sense acceleration or vibration perpendicular to the
plane of the loop. Optionally, a mass 63 may be attached near the
end of the structure to modify the dynamic response of the sensor.
The sensor could be used to perform a wide variety of acceleration
measurements, including measurement of impact deceleration for the
purpose of deploying an airbag protective system for automobiles.
This structure is amenable to being manufactured by micromachining
of a semiconductor or glassy substrate, wherein the fiber would be
formed from the substrate and undercut by an etching process. A
thin web or plate affixed to one side of the loop (not shown) could
be added to prevent the shape of the loop from changing within its
plane.
FIG. 36 portrays a displacement sensor using a loop sensor 68
consisting of a fiber 51 with a treated portion 52 on a loop at its
distal extent, attached to a spring at pivot point 64. The sensor
is designed to sense a large displacement 67 of structure 66
relative to structure 65, which is fixed to a frame of reference to
which the fiber is also fixed, both by means of holding structures
57. By selecting the attachment point of pivot point 64, the
movement of the end of the sensor is restricted to a smaller range
than movement range 67. The fractional amount of reduction is
according to the ratio of distance X2 to the total of distances X1
and X2, yet the reduced movement is linearly related to the
movement of structure 66. The pivot point may take the form of a
hinge, ball joint, flexible beam, cable, wire, elastomer, or
various sliding contact points, or alternatively, the loop sensor
may be mounted inside the spring or through the turns of the spring
in various ways such that its curvature is linearly related to the
linear movement of the pivot point. This embodiment allows the use
of a small curvature sensor with limited travel to measure large
displacements.
It will be seen that the sensors described are particularly
convenient for being embedded in structures. The loop sensors are
particularly suited for measurements in thin tubes, pipes, rods and
the like, particularly if measurements must be performed near the
ends of the member. The loop sensors are well adapted for use in
various probes that must be inserted into small spaces.
Applications for the sensors in general are meant specifically to
include at least all applications that could potentially be
performed with strain gauges, plus others.
The sensor may be incorporated into a means of transport, means for
construction, agricultural implement, robot, living body or a
prosthetic device, for detecting a movement relation to a further
movement or a point of reference.
FIG. 37, which is similar to FIG. 14, illustrates one example of
electronic circuitry that can be used to measure the transmission
of light through a paired sensor element such as that shown in
earlier Figures. In FIG. 37, fiber 39' (shown coiled to indicate
arbitrary placement and length of the guide conveying light to and
from the sensing portion) has the light emitting strip at 30'.
Fiber 45', which is otherwise the same as fiber 39', has no sensing
portion at position 46', which represents a section of the fiber in
close proximity to sensor section 30'. Both fibers are illuminated
by photoemitters E1 and E2, which are light emitting diodes.
Photodetectors D1 and D2, which receive light from the fibers, are
PIN photodiodes, backbiased with -12 Volts to enhance the speed of
their response to light energy. U1 and U2 are high input impedance
operational amplifiers arranged as transimpedance amplifiers,
converting light energy linearly into voltages fed to the inputs of
U3, which is an operational amplifier connected as a differential
amplifier with a gain 10. The gain of amplifier U2 can be varied
with R1 so that for a straight fiber, the inputs to U3 are equal.
In this condition, the optoelectronic circuit is analogous to a
two-armed bridge such as is used to make strain-gauge measurements.
Errors due to degradations in the fibers, connector variations,
temperature fluctuations, and the like tend to cancel before
reaching the output of U3. The output of U3 is a voltage which
varies with bending at the band portion 30'. The output voltage can
be further amplified and sent to a display unit or used to control
various parameters, such as actuators designed to minimize the
angle of bend.
Many variations of the circuitry are possible, including variations
with greater immunity to error sources. One such variation would
use the same light source and detector, separating the signals by
chopping them at different frequencies and employing synchronous
detection. Another variation is to replace U3 in the above Figures
with a divider circuit, so that the sensor signal is divided
arithmetically by the reference signal.
Another variation uses one photoemitter, for example E1 in FIG. 14,
to illuminate the sensor fiber, such as 19 in FIG. 14, and also to
illuminate a reference fiber such as 25 in FIG. 14. This last
variation may be further enhanced by eliminated U3, using U2 and
another amplifier to control the light out of 25 to a constant
value, and reading relative light transmission through 19 directly
from the output of U2.
FIGS. 38, 39 and 40 illustrate other forms of fiber sensors having
normal circular cross-sections.
In FIG. 38 a sensor 70 is formed from a flat strip having a
rectangular cross-section. It has two opposed wide flat surfaces or
sides 71 and 72. The strip is bent into a U-shape, in the Example,
in a plane parallel to the flat surfaces 71 and 72. A light
emission surface 73 is formed on one of the flat surfaces, surface
71, at the curve 74, and in use a sensor deflects in a direction
normal to the flat surface 71 as indicated by arrow F. This
deflection or curvature is out of the plane of the fiber, and is
illustrated in FIG. 38B.
In FIG. 39A, the sensor 70 is again formed from a flat strip having
a rectangular cross-section, with opposed, spaced-apart, wide flat
surfaces or sides 71 and 72. In this Example the strip is bent into
a U-shape in a plane normal to the planes of the surfaces 71 and
72. Light emission surfaces 75, 76 are formed on each of the
surfaces 71 and 72, adjacent to the bend 77. The emission surfaces
face in the same direction, as seen in FIG. 39C particularly. The
deflection is illustrated in FIG. 39B.
FIG. 40A illustrates a fiber sensor 80, with a D-shaped
cross-section as seen in FIG. 40B. In this Example, the fiber is
bent in a U-shape, in a plane parallel to a flat surface 81 of the
fiber, the bend indicated at 82. A light emission surface 83 is
formed on the flat surface 81 at the bend 82. The deflection of the
sensor would normally be downward out of the plane of the surface
81, as indicated at the dotted outline 80A.
Rectangular cross-sections, or similar cross-sections, have certain
advantages. They can provide very efficient modulation of light by
bending, as a fiber having one predominant mode can be used.
Virtually all of the light can be made to intercept the emission
surface.
FIG. 41 illustrates another sensor made of a flat strip having a
rectangular cross-section. It has two opposed wide flat surfaces 71
and 72 and a light emission surface or surfaces 84 and 85 on the
same surface, in this case surface 71. In this example the strip is
bent into a U-shape but in such a way that the long axis of its
cross-section is perpendicular at section Q-Q' compared to the
orientation of said long axis at section R-R'. To maintain it in
this shape but with minimal effect on bending in a plane
perpendicular to the plane of the U-shape, a thin rigid bar or bars
86 may be attached to the sensor adjacent to where the orientation
of the cross-section begins to change.
FIG. 41' is a graph of the light throughput of a sensor like the
one in FIG. 41. In this case the sensor was made of a strip of
optically clear polymer, with a cross-section of 0.18 mm.times.1 mm
and a total length of 130 mm. The rigid bar had a cross-section of
0.8 mm.times.0.2 mm and was 3 mm long. It was placed with its
greatest width flat against the optical strip, 4 mm from the apex
of the loop. The structure was held in a fixture, such that the
loop was free to flex out of the plane of the loop in a roughly
circular arc. The horizontal axis in the Figure indicates curvature
of the loop, expressed as a total angular deflection from a planar
(non-deflected) loop, per 4 mm (the approximate length of the
region of the loop allowed to flex, measured along the axis of
symmetry of the loop). The vertical axis indicates light
transmission through the light guide expressed as a percentage of
the maximum transmission measured during the test, where zero
transmission corresponds to no light travelling through the light
guide. The graph shows the ability to achieve a sensor with a
linear range which extends substantially into a region of curvature
such that increasing curvature produces increasing light
transmission.
FIG. 42 shows a curvature or displacement sensor 87 on fiber 89
affixed to a bending beam 88. The bending beam is situated with one
end 90 attached to a reference structure 91. Arm 92 pivots relative
to the reference structure on hinge 93. Spring 94 is attached under
tension to the pivot arm by means of pin attachments 95. This
arrangement is used to measure the pivot angle of arm 92 by means
of transmitting its angular displacement to the bending beam 88, in
such a way that extraneous movement of pivot arm 92, such as
end-to-end displacement, minimally affects the curvature of said
beam. Alternatively, the spring may be replaced by a shear pad or
other structure that minimizes the number of mechanical modes
transmitted to said beam. This method of attachment is useful in
applications such as automotive suspension height measurement and
is preferable to the use of a secondary bean at right angles to
primary beam, as in Y and T.
In the case of light guides with a cross-section that is
rectangular or otherwise non-circularly symmetric, if the modes
entering the loop (not counting the mode-modulating effects of the
surfaces) are weighted toward being in the plane of the loop, then
it is best to apply bends to the assembly within the plane of the
loop. This is normally also the plane of greatest flexibility for
deflections. The emission surfaces then logically must be
perpendicular to the plane of the loop, and must face such that all
undergo curvatures of the same sign. In this case this is the
optimum configuration because the predominant modes are those
affected by the modulation, regardless of loop geometry. In the
case of non-circularly symmetric light guides where one mode
substantially outweighs all others, there is little chance of
mode-mixing downstream of the emission surface or surfaces, so that
the surfaces need not be placed on or near the loop to achieve
optimum modulation. An example of such a sensor light guide would
be a thin strip of clear optical polymer with a rectangular
cross-section, such that the major axis of the cross-section is
perpendicular to the plane of the loop. If the loop is in the plane
of the paper, apex facing right, then all the emission surfaces
would be placed on sides of the light guide which face, e.g., the
top (alternatively all face the bottom) of the paper.
The response of sensors is explained in the following way:
Light rays travel along fiber optics throughout a range of angles
limited by the difference in index of refraction between the core
and the cladding. For straight fibers, some light rays pass through
the emission band. As a fiber with the emission band at the top
bends downward, more of the rays impinge on the band at angles
capable of passing through the surface, either by diffusion or by
direct transmission. As the fiber bends upward, fewer rays will
impinge on the band, and will be at shallow angles to its surface,
so that more of them stay within the fiber, refracting from the
untreated core/cladding boundary toward the fiber outside the
sensor region. Bends at right angles to the axis of maximum
sensitivity change to a minimum degree the amount of light striking
the emission band, so there is virtually no change in transmitted
light.
The width of the emission band around the circumference of the
fiber will determine the sensitivity of the fiber to bends, with
wider bands producing a larger percentage change per degree of
bend. However, very wide bands will tend to increase the
sensitivity to bends in the axis of minimum sensitivity. Typical
sensors have emission bands that cover 5.degree. to 30.degree. of
the circumference of the fiber, but other values will work. The
length of the emission band can vary. It can be any length from
millimetres to meters but there is less gain in sensitivity for
long lengths, than expected. There is an optimum length for the
strip, which is a function of the diameter of the fiber, its
emission band width, and the minimum linear deflection angle
desired. For 1 mm fibers, the optimum is about 25-50 mm, and for
200 micron fibers, it is about 10-20 mm. Long sensors can be formed
by alternating lengths of fibers with an emission strip with
lengths of fully clad fibers. Short sensors will be less sensitive,
but more specific as to location of bend along the length of the
member to which they are attached. However very short sensors can
be made, such as 8 mm sensors on 125 micron fibers.
Sensors in accordance with the invention have various advantages,
and useful characteristics. No special electronics are required to
measure interference patterns, it is only necessary to measure the
amount of light transmitted. Cost is orders of magnitude below that
for interference (OTDR) techniques. Sensitivity is in the same
range as that of resistance strain gauges. For many situations,
particularly when the sensors are mounted near the neutral axis of
a bending beam, changes in signal per microstrain are greater than
those for resistance strain gauges, as measured in percent. The
linear range is very large. They are particularly suited to
measurement of bending in aircraft wings, helicopter blades,
machinery, robot arms, or large structures. They are not affected
by temperature, since measurement is not dependent on small changes
in length of the fiber or its sensing portion. This is a distinct
advantage over resistance strain gauges, which have a relatively
narrow range of temperature sensitivity unless compensated, and
over interference techniques, which are affected even more by
temperatures than are resistance gauges. They can be used to
measure position, with a large dynamic range.
While the above are descriptions of various preferred embodiments
of the invention, various modifications should be obvious to those
skilled in the art. For example, the sensor may be used with
virtually any wavelengths or light including broadband or discrete
spectra. Various referencing methods may be used, including a
separate untreated fiber that is used to send light from the sensor
light source over a similar path as the sensor fiber, the intensity
of said light being used to perform differential or ratio
compensation for common mode errors such as variations in fiber
transmission or light source intensity. The reference path may also
be used to automatically adjust the source light intensity to a
fixed level. The invention is also meant to allow for emission
surfaces that allow light loss at one wavelength or band of
wavelengths but not at another wavelength or band of wavelengths.
Thus, a two wavelength referencing method could be used over the
same fiber, wherein light loss due to curvature is sensed with one
wavelength and the other is used to provide reference information
for compensation for common mode errors. Also, the light path may
be chopped or modulated to provide improved sensitivity, if
necessary. The output of the sensor may be used to measure
parameters over a large range, or to determine a switching level.
The sensors may be used for measurement, or form a part of a
control loop. The sensor fiber may be attached to a substrate that
is undergoing bending, or the fiber alone, supported by two or more
parts whose deflection is being measured, may be the element being
bent. Consequently, the description should not be used to limit the
scope of the invention.
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